Aluminum and its alloys are particularly useful materials for inclusion in metal components of vehicles such as cars, trucks, airplanes, and the like. Aluminum alloys are lighter than steel alloys and thus offer weight advantages in many applications in vehicles. The light weight and excellent heat transfer properties of aluminum alloys make them particularly attractive candidates for use in heat exchangers such as radiators, heaters, evaporators, oil coolers, condensers and the like. These heat exchangers and similar components are typically fabricated from a multitude of formed or extruded parts that are subsequently assembled, fixtured, cleaned and joined together in a brazing process. In brazing of aluminum work pieces, an aluminum brazing alloy (e.g., an aluminum-silicon alloy) is positioned between the surfaces to be joined and the work pieces are heated to a temperature which melts the brazing alloy but not the underlying work piece. Upon cooling, the brazing alloy solidifies as a joint between the work pieces. The brazing alloy is typically introduced onto the surfaces of aluminum stock by cladding thereto in a roll bonding operation.
A common brazing practice includes cleaning of the components via a suitable solvent (to remove oils and the like from the surfaces to be brazed) followed by application of a flux to the pre-brazed components to be joined. The fluxed components are heated in a controlled atmosphere to retard oxidation, this atmosphere being typically dry nitrogen. The role of the flux is to reduce the oxides on the faying surfaces of the components which are to be joined via brazing. The flux is applied after fabrication of the individual work pieces to be brazed, commonly after assembly of the components (e.g. as a heat exchanger) prior to brazing. The flux may be applied directly as a dry powder or mixed with a carrier such as water or alcohol and applied as a slurry over the entire work piece. In the latter case, the carrier is subsequently removed via a drying step, leaving the flux as a powder on the surface of the work piece.
The flux is only required in areas where metallurgical bonds or joints are required. Nevertheless, it is common manufacturing practice to apply flux over the entire assembly, often including the fixtures used to contain the parts during the brazing step in the furnace. This results in overuse and waste of flux, the need to clean the fixtures and increased maintenance of the furnace due to the corrosive nature of flux. Moreover, the processes of cleaning and applying flux are time consuming and concomitantly expensive. It should be further noted that the flux is loosely adhered to the work pieces as a powder. Hence, care must be taken to avoid removal of the flux during any handling of the components prior to brazing.
An alternative to fluxing the entire assembly is to apply flux to the work pieces prior to working or forming the material in a pre-fluxing operation. Pre-fluxing is advantageous in that the flux can be applied only on the cladding where joints are formed; unclad areas are without flux. However, conventional pre-fluxing techniques have not found broad commercial applications.
One pre-fluxing method has been to disperse flux in a binder and coat the work piece with the flux-binder mixture. During brazing, the binder volatilizes which may results in undesirable voids within the joint that must be filled to ensure sealing of the brazed components Another drawback to this flux-binder coating technique is that the brazing surfaces typically must be cleaned beyond standard rolling mill cleanliness standards thereby increasing the operating costs by several cents per pound of brazing metal produced.
An alternative route to pre-fluxing is to eliminate the cladding process and apply flux and a cladding metal or alloy in deposition processes either simultaneously or sequentially. One such technique is thermal spraying as disclosed in U.S. Pat. No. 5,594,930. The '930 patent teaches spraying molten droplets of aluminum and silicon or an alloy thereof onto a brazeable aluminum substrate. U.S. Pat. No. 5,820,939 also discloses a method of thermally spraying metallic coatings on unroughened cleaned aluminum alloy substrates. The method includes wire-arc thermally spraying of melted metallic bonding droplets and fluxing particles onto the substrate using gas propulsion to concurrently deposit flux particles and bonding droplets. In these methods, molten droplets pass through air and form additional oxides thereon which compounds the need to deoxidize the substrate.
Hot pressing of powders of aluminum, silicon or an alloy or mixture thereof onto an unclad aluminum substrate is described in U.S. Pat. Nos. 5,330,090 and 5,547,517. Compaction of powders typically results in minimum void levels of about ten percent. Voiding is undesirable and the process of hot pressing the powders onto the substrate can be cumbersome.
Coating processes for simultaneous application of flux with aluminum and silicon are described in U.S. Pat. Nos. 5,100,048 and 5,190,596. The '048 patent teaches a process of dipping unclad aluminum substrate into an alcohol slurry of aluminum, silicon and flux. Upon evaporation of the alcohol, the silicon and flux remaining on the substrate is weakly adhered thereto and tends to spall off the substrate during assembly. The '596 patent discloses a method of applying a paste containing aluminum, silicon and a binder onto unclad aluminum substrate. In either case, the silicon and aluminum form a thin clad layer on the aluminum substrate and a flux is incorporated therewith. This system adheres better to the substrate, but the volatilized binder creates voids in the joint.
Accordingly, a need remains for a method of depositing brazing flux onto metal substrates prior to working of the metal which minimizes the amount of flux used in the brazed assembly, adheres flux to the substrate without the use of a binder, and may additionally deposit metal cladding into the substrates.